Huang Xiaoting, Gao Yang, Yang Tianqi, Ren Wencai, Cheng Hui-Ming, Lai Tianshu
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun Yat-sen University, Guangzhou 510275, China.
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China.
Sci Rep. 2016 Aug 31;6:32236. doi: 10.1038/srep32236.
We report the temperature-dependent evolution of Raman spectra of monolayer WS2 directly CVD-grown on a gold foil and then transferred onto quartz substrates over a wide temperature range from 84 to 543 K. The nonlinear temperature dependence of Raman shifts for both and A1g modes has been observed. The first-order temperature coefficients of Raman shifts are obtained to be -0.0093 (cm(-1)/K) and -0.0122 (cm(-1)/K) for and A1g peaks, respectively. A physical model, including thermal expansion and three- and four-phonon anharmonic effects, is used quantitatively to analyze the observed nonlinear temperature dependence. Thermal expansion coefficient (TEC) of monolayer WS2 is extracted from the experimental data for the first time. It is found that thermal expansion coefficient of out-plane mode is larger than one of in-plane mode, and TECs of and A1g modes are temperature-dependent weakly and strongly, respectively. It is also found that the nonlinear temperature dependence of Raman shift of mode mainly originates from the anharmonic effect of three-phonon process, whereas one of A1g mode is mainly contributed by thermal expansion effect in high temperature region, revealing that thermal expansion effect cannot be ignored.
我们报道了直接在金箔上化学气相沉积（CVD）生长然后转移到石英衬底上的单层WS₂在84至543 K的宽温度范围内拉曼光谱随温度的演变。观察到了E₂g和A₁g模式的拉曼位移的非线性温度依赖性。对于E₂g和A₁g峰，拉曼位移的一阶温度系数分别为-0.0093（cm⁻¹/K）和-0.0122（cm⁻¹/K）。使用包括热膨胀以及三声子和四声子非谐效应的物理模型对观察到的非线性温度依赖性进行了定量分析。首次从实验数据中提取了单层WS₂的热膨胀系数（TEC）。发现面外模式的热膨胀系数大于面内模式的热膨胀系数，并且E₂g和A₁g模式的热膨胀系数分别对温度有弱依赖性和强依赖性。还发现E₂g模式的拉曼位移的非线性温度依赖性主要源于三声子过程的非谐效应，而A₁g模式的非线性温度依赖性在高温区域主要由热膨胀效应贡献，这表明热膨胀效应不可忽略。
